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P and q tubes fill a cistern in 12 minutes and 15 minutes, respectively. They were opened together and the first tube was closed 3 minutes after filling. How long was the empty cistern full?

Sagot :

Hello!

First, we have to calculate the least common multiple of 12 and 15.

LMC(12, 15) = 60

So, we discovered that the capacity of the cistern equals 60 liters.

Now, let's calculate how many liters of water each tube can fill:

P: 60/12 = 5 liters per minute

Q: 60/15 = 4 liters per minute

Analyzing the exercise:

"They were opened together and the first tube was closed 3 minutes after filling"

As we know when the first tube (P) was closed, let's calculate how much water it filled:

5 liters per minute * 3 minutes = 15 liters of water

Also, let's calculate the same with the second tube (Q) because they're filling together:

4 liters per minute * 3 minutes = 12 liters of water

So, we know that after 3 minutes of filling, the cistern has 15 + 12 = 27 liters of water, right?

Let's calculate how many liters the cistern needs to be full:

60 - 27 = 33 liters

Remember: Now, just tube Q is filling.

33 liters divided by 4 liters per minute:

33/4 = 8.25 minutes remaining to the cistern be full